Extracting extra-heavy oils or bitumen from the known oil sands or oil shale formations requires their flowability to be increased substantially. This can be achieved by increasing the temperature of the formation (reservoir).
The most widely established and applied in-situ process for extracting bitumen or extra-heavy oil is the SAGD (Steam Assisted Gravity Drainage) method. This entails forcing water vapor under high pressure through a pipeline (well) running horizontally inside the seam. The heated, molten bitumen or extra-heavy oil separated from the sand or rock percolates down to a second pipeline or well located approximately 5 m deeper, through which the liquefied bitumen or extra-heavy oil is extracted, the spacing between injector and production pipeline or well being dependent on the reservoir geometry.
With this system, the water vapor has to fulfill a number of tasks simultaneously, namely introducing the thermal energy required to produce the liquefaction, separating out the bitumen or oil from the sand, and building up the pressure in the reservoir in order on the one hand to make the reservoir geomechanically permeable for bitumen transportation (permeability) and on the other hand to enable the bitumen to be extracted without additional pumps.
The SAGD method starts with steam being introduced through both pipelines or wells for a period of, for example, three months in order first to liquefy the bitumen in the space between the pipelines or wells as quickly as possible. Thereafter the steam is injected through the upper pipeline or well only and the extraction through the lower pipeline or well can begin.
It is already disclosed in the German patent application DE 10 2007 008 292 A1 that the SAGD method conventionally used for this purpose can be complemented with an inductive heating device. Furthermore, the German patent application DE 10 2007 036 832 A1 describes a device in which parallel running inductor or electrode arrangements are present which are connected above ground to an oscillator or inverter.
In the earlier German patent applications DE 10 2007 008 292 A1 and DE 10 2007 036 832 A1 it is therefore proposed to overlay the injection of steam with inductive heating of the deposit. In the process resistive heating between two electrodes may also take place in addition under certain conditions.
With the above-described arrangements the electrical energy must always be conducted by way of an electrical outgoing conductor and an electrical return conductor. A not inconsiderable investment in terms of effort and cost is required for this.
In the cited earlier patent applications, individual inductor pairs consisting of outgoing and return conductor or groups of inductor pairs in different geometric configurations are supplied with electric current in order to inductively heat the reservoir. In this case it is assumed that there is a constant spacing between the inductors within the reservoir, which, given a homogeneous distribution of electrical conductivity, results in a constant heating power being output along the inductors. Described therein are the outgoing and return conductors routed spatially close together in the sections in which the overburden is penetrated in order to minimize the losses there.